This invention relates to a wet developing apparatus used in electrophotographic copying machine for the development of electrostatic latent images using a liquid developer.
For the development of electrostatic latent image (hereinafter called as "latent image") formed on a electrophotographic photosensitive sheet (hereinafter called as "photosensitive sheet") using a liquid developing apparatus for electrophotographic copying machine, it is generally recognized that the following requirements are indispensable to obtain a clear and sharp reproduced image:
(I) The photosensitive sheet is permitted to come nearer to a developing electrode;
(II) Even and adequate development is obtained on the surface of a latent image;
(III) Back side of the latent image is free from any stain by toner;
(IV) The photosensitive sheet is assured of smooth advance.
With respect to the foregoing requirements, it is usually understood that following points are to be taken into consideration:
(a) When the developing electrode comes nearer to the photosensitive sheet, a resolving power is increased, a high density being obtainable, and a halo, i.e., an edge effect peculiar to the electrophotography being decreased;
(b) To obtain even and adequate developing density, it is required not to produce a variation in the local electric characteristics (resistance, for example) caused by inadequate or non-uniformed spacing between the electrodes or by the partial stain, and also required to feed the developing solution uniformly and sufficiently onto the surface of the photosensitive sheet;
(c) To prevent the back side of the photosensitive sheet from being stained by toner and to avoid disordered electric charge on the photosensitive sheet, the back side shall not be put in contact with an auxiliary electrode plate;
(d) To ensure smooth advance of the photosensitive sheet, a guide of low frictional resistance is required.
A number of attempts have been proposed to meet the above-discussed requirements (a), (b), (c) and (d) at a time. Indeed that each of those proposals has its own advantage, but that the requirements for the fair reproduction (a reproduced image of high density, noiseless and high contrast, for example) and for the prevention of the back side from being stained have not been satisfactorily met yet.
According to one example disclosed in Japanese Patent Application laid open under Provisional Publication (unexamined) No. Sho 49-62148, in order to meet the foregoing requirements (I) through (IV), a developing solution is applied to both sides of the photosensitive sheet, a nylon thread having larger electric resistance than that of the developing solution is stretched facing to the auxiliary electrode plate, and the photosensitive sheet is positioned using the nylon thread as a guide. According to said apparatus, as the photosensitive sheet is immersed or bathed in the developing solution, the enforcement to be operated to the toner becomes less or reduced, and thus the characteristics proper to the toner and those of the latent image are appropriately represented as it is. As the result thereof, a relatively successful density, contrast, etc. are obtained and the stain on the back side is not so much. According to said apparatus, however, in order to make a rapid progress of the development, it is necessary to accelerate the polarization in the developing solution thereby the external effect of the electric field of the latent image being increased. Thus it will be essential to use a developer of lower resistance, as the result of which edges of the image are disordered inviting such disadvantages as sagging of edge, mal-reproduction of details. Furthermore, the back side is not completely free from stain because of the existence of gut or nylon thread, and in particular the back side of blackened part is easy to be stained. Besides, in case of using said apparatus over relatively longer period (several weeks), the toner sticks to the contact area between the gut and the auxiliary electrode plate, and it becomes necessary to carry out periodical maintenance service such as cleaning of the auxiliary electrode plate, which is a troublesome work.
According to another example disclosed in Japanese Utility Model Application published under Publication (examined) No. Sho 56-14525, an apparatus is proposed having electroconductive meshes stretched opposite to the auxiliary electrode plate as a guide as well as for positioning, and making use of so-called an effect of electro-conductive mesh. According to an experiment, however, it is found that said effect of electro-conductive mesh has following disadvantages:
(i) When the developing solution is not fed flowing through the back side of the photosensitive sheet, it is difficult to obtain an adequate developing density;
(ii) Back side stain cannot be prevented even by using a fine mesh of about #300, for example;
(iii) Toner sticks to the meshes during the period of repeated uses and the sticked toner can hardly be removed when dried. Particularly at the first copying on each day, the back side is stained by local uneveness of image density and by the physical contact. Furthermore it is quite troublesome to periodically clean the elector-conductive meshes placed on the auxiliary electrode plate.
According to a further example published by U.S. Pat. No. 3,547,076, which makes use of so-called Al effect of electrical field, the developing density is increased by voltage elements which push away the toner to the side of latent image, and the toner sticked to the non image area on the surface of the photosensitive sheet causing a noise on the back side thereof can be removed or separated therefrom by the voltage elements of reversed polarity. Said apparatus, however, can perform such advantage in a quite limited circumstance or only when the photosensitive sheet applied thereto has an electro-conductive backing of which resistance is sufficiently low. Accordingly, either in case of some photosensitive sheet without such electroconductive backing and necessary to be charged by means of corona discharge or in case of the other photosensitive sheet of high resistance, the impressed AC electrical field not only increases the developing density on the image surface but also accelerates the sticking of toner to the back side of the latent image, resulting in considerable stain thereof.
In view of the above-discussed disadvantages, the applicant has proposed a liquid developing apparatus satisfying the aforementioned requirements (I) to (IV) as is disclosed in Japanese Patent Application No. Sho. 58-116709.
To be more specific, the applicant disclosed a liquid developing apparatus used for electrophotography comprising a passage for feeding developing solution which is formed between a main electrode plate and an auxiliary electrode plate facing to each other so that a photosensitive sheet having an electrostatic latent image therein is developed in said passage, wherein at least a surface of said auxiliary electrode plate facing to the back side of the photosensitive sheet is formed into a roughened surface.
Even in case of such improved apparatus, there exists a problem of wear and tear described hereunder.